1. Field of the Invention
The present invention relates to pivot links, such as that by which the rocker lever of a fuel injector drive train acts on the upper plunger of a fuel injector. In particular, the invention relates to such pivot links in which the pivot surface is formed on an element made of a ceramic material.
2. Description of Related Art
Links of the type to which the present invention is directed are comprised of shaft and ball surface portions and act to transfer compressive loads between contact surfaces at a sliding interface without bending moments. Such links are commonplace in engine subsystem drive trains, e.g., valve and fuel injector drive trains, and to reduce wear at the sliding interface, recent developments have been directed toward making the ball surface portion(s) of a ceramic material. For example, U.S. Pat. Nos. 4,806,040; 4,848,286; 4,966,108; and 5,083,884, as well as Japanese Patent Application 57-13203 disclose pivot rods in which the pivot surface is provided by a ceramic element that is fixed to the end of a shaft by such techniques as interference fitting, diffusion bonding and deforming of a sleeve.
However, the performance and durability of links and link joints are largely dependent on the geometric precision (i.e., size, form and finish) of the mating surfaces. Operating stresses, wear and travel (lash) caused by wear are all directly related to dimensional uncertainties (manufacturing tolerances) of critical feature characteristics, such as radius, radius offset, profile, and surface finish. Furthermore, links and link joints are generally designed at the limits of manufacturing and inspection capabilities making the incremental cost of improving geometric precision great, especially when ceramic materials are used, due to the difficulties encountered in machining and non-destructively evaluating component integrity. Bearing type balls made of ceramic materials, e.g., silicon nitride, are commercially available; but, to date, no arrangement has been developed which offers a low cost means of incorporating a free-rolling ceramic ball into a precision component, such a pivot link of the type to which this invention is directed.
U.S. Pat. No. 4,141,329 shows a fuel injection system in which a cam acts on a ball which is rotatable in a socket in the end of a piston stem cap, but such an arrangement cannot be used to provide a ball at both ends of a link shaft and since the ball is not positively retained in its socket by anything other than the force of a spring used to urge the stem cap toward the cam, should lash develop with wear, the potential exists for the ball to become dislodged from the stem cap with possibly disastrous results. U.S. Pat. No. 2,636,757 discloses a ball and socket joint for an engine valve drive train in which a free ball is disposed between a pair of sockets that are coupled by a housing that encloses the ball. However, this arrangement is impractical since the housing coupling the pair of sockets and enclosing the free ball inhibits the pivotal movement that the free ball is intended to facilitate.
Thus, there is still a need for an improved pivot link arrangement by which a free ball can be retained on one or both ends of a pivot link, especially so as to enable a pivot link to be provided with a ceramic ball surface in a way that will reduce manufacturing costs operating stresses, wear and travel (lash).